Many industrial heat treating processes involving, for example, metals and glass, utilize gas-fired rapid heating as a means of achieving desired surface properties. The use of gas-fired rapid heating techniques for metal and glass products has many advantages over typical furnace heating techniques, namely: high thermal efficiency, improved product quality, faster heating response time, and increased productivity. During these processes, a flame or jet of hot combustion products directly impinges upon the object to be heated. Such impingement heating eliminates the need for large radiative furnaces, since up to 80 percent of the heat transfer occurs by convection. Also, greater control of the heat transfer process may be possible, due to the rapid response time of convective heating.
Two major problems are associated with the use of flame jet impingement for heat treatment processes. First, all of the data in the open literature are for round impinging in-line jet (ILJ) flames. The ability of these jets to heat a surface is highly dependent upon the nozzle Reynolds number and the nozzle-to-surface spacing (X). For pure diffusion flames, large X values (X/D.sub.r &gt;40) are required for the maximum heat flux to be achieved. This is due to the fact that ample residence time is required for the fuel to mix with surrounding air and burn prior to impingement upon the surface. Pre-mixing the fuel and air eliminates this problem and can improve the heat transfer, but most industrial processes tend to avoid full pre-mixing of fuel and air, due to the possibility of accidental flashback and exploding. Therefore, a type of nozzle that produces a partially pre-mixed flame that will also provide symmetric surface heating is utilized. One such nozzle is a Radial Jet Reattachment (RJR) nozzle, which is described in Page, R. H., Hadden, L. L., and Ostowari, C., 1989, "Theory for Radial Jet Reattachment Flow," AIAA Journal, Vol. 27, No. 11, pp. 1500-1505, which is incorporated herein by reference.
Radial Jet Reattachment nozzles have seen extensive use in non-reacting jet impingement heat transfer and drying processes. The aerodynamics of the RJR nozzle differs greatly from a standard ILJ nozzle, and this RJR flow pattern has been utilized to design a single Radial Jet Reattachment Combustion (RJRC) nozzle, which is described in Habetz, D. K., Page, R. H., and Seyed-Yagoobi, J., 1994, "Impingement Heat Transfer from a Radial Jet Reattachment Flame," 10th International Heat Transfer Conference, Brighton, U.K., Vol. 6, pp. 31-36, which is incorporated herein by reference. Although an RJRC nozzle produces a partially pre-mixed flame, a single RJRC nozzle may not be adequate to produce the desired results. However, no data exists regarding the performance of radial jet reattachment nozzles in an array configuration.